A variety of mount assemblies are presently available to isolate vehicle vibrations, such as for automobile and truck engines and transmission. One of the most popular mounts today is the hydraulic-elastomeric mount of the type disclosed in U.S. Pat. No. 4,588,173 to Gold et al., issued May 13, 1986, entitled "Hydraulic Elastomeric Mount" and assigned to the assignee of the present invention.
The hydraulic mount assembly of this prior invention includes a reinforced, hollow rubber body that is closed by a resilient diaphragm so as to form a cavity. This cavity is partitioned by a plate into two chambers that are in fluid communication through a relatively large central opening or passage in the plate. The first or primary chamber is formed between the plate and the body. The secondary chamber is formed between the plate and the diaphragm.
A decoupler is positioned in the central passage of the plate and reciprocates in response to the vibrations. The decoupler movement alone accommodates small volume changes in the two chambers. When, for example, the decoupler moves in a direction toward the diaphragm, the volume of the portion of the decoupler cavity in the primary chamber increases and the volume of the portion in the secondary chamber correspondingly decreases and vice versa. In this way, for certain small vibratory amplitudes and generally higher frequencies, fluid flow between the chambers is substantially avoided and undesirable hydraulic damping is eliminated. In effect, this freely floating decoupler is a passive tuning device.
In addition to the relatively large central passage, an orifice track with a smaller restricted flow passage is provided extending around the perimeter of the orifice plate. Each end of the track has an opening; one opening communicating with the primary chamber and the other with the secondary chamber. The orifice track provides the hydraulic mount assembly with another passive tuning component, and when combined with the freely floating decoupler, provides at least three distinct dynamic operating modes. The particular operating mode is primarily determined by the flow of fluid between the two chambers.
More specifically, small amplitude vibratory input, such as from relatively smooth engine idling or the like, produces no damping due to the action of the decoupler, as explained above. In contrast, large amplitude vibrating input, such as when the engine is excited at its resonant frequency or when the vehicle suspension inputs large displacements (e.g. sudden acceleration or panic stop), produces high velocity fluid flow through the orifice track, and, accordingly, a relatively high level of damping force and desirable smoothing action.
A third or intermediate operational mode of the mount occurs during medium amplitude inputs experienced in normal driving and resulting in lower velocity fluid flow through the orifice track. In response to the decoupler switching from movement in one direction to another in each of the modes, a limited amount of fluid can bypass the orifice track by moving around the edges of the decoupler and through the central opening, thereby smoothing the transition.
This basic mount design has proved quite successful and represents a significant advance over the prior art engine mounts and particularly those of the solid rubber type. More specifically, hydraulic mounts provide a more favorable balance of load support and damping control. It should be appreciated, however, that additional improvement in operating characteristics is possible, and indeed, to a significant degree, substantial progress has been made recently.
More recent developments in hydraulic mount technology have led to the advent of electronic control of the dynamic characteristics of the mount. Advantageously, such a mount allows active rather than passive control. Thus, more efficient and effective isolation of vibration and suppression of noise may be provided. A previously developed hydraulic mount of the active control type is disclosed in U.S. Pat. No. 4,783,062 to Hamberg et al., issued Nov. 8, 1988, entitled "Electronic Hydraulic Mount-Internal Solenoid" and assigned to the assignee of the present invention.
In this mount assembly the partition includes at least two passages connecting the primary and secondary chambers. One of the passages may be a central opening but no decoupler is specified in the preferred embodiment. A sliding gate extends across the entry to the central opening. Two other passages of varying length form independent orifice tracks providing unique damping characteristics tuned to isolate selected frequencies of vibration and provide the desired engine control. This gate is displaceable to direct the flow of fluid between the primary and secondary chambers through a selected passage or passages in the partition.
A solenoid actuator mounted on the partition includes multiple electric coils that allow the positive positioning of the gate. A control circuit with on-board transducers is provided to monitor vehicle operating and road conditions. A microprocessor acts in response to the sensed conditions causing the necessary sequential energization of the series of coils to properly position the gate and provide the desired damping characteristics.
The mount assembly described in the Hamberg et al. patent is particularly adapted for tuning to the resonance frequencies characteristic of the vehicle component being damped. This allows the mount assembly to more efficiently and effectively isolate vibrations and suppress noise over a wide range of vehicle operating and road conditions.
While the mount assembly disclosed in the Hamberg et al. patent may be very effectively tuned to provide the desired damping characteristics, still further progress and improvements in the active mount assembly design are possible. More particularly, it is now contemplated to provide a mount assembly that incorporates the best of both active and passive tuning features. In doing so, for simplicity of design and low cost, the basic decoupler design, plural orifice tracks and solenoid control that have been used in the past and are proven to be reliable in operation, are selected to be included. However, to provide operating characteristics more suited to particular applications and enhance the efficiency for a still wider range of vehicle operating conditions, the manner in which these features are combined is responsive to solenoid operation.
In this regard, it is particularly desirable to provide the improved mount assembly with up to five distinct operating modes rather than the standard three, each mode having enhanced operating efficiencies. In a first operative position of a control element, the mount assembly should provide the three standard or normal passive modes of operation; i.e. the same proven operative modes provided by the mount assembly disclosed in U.S. Pat. No. 4,588,173 to Gold et al. (as described above) are to be brought into play.
More specifically, in the first mode, the decoupler is to reciprocate without seating in response to low or small amplitude, generally higher frequency vibrations occurring during normal vehicle idling or other low load operation. Small volume changes in the two chambers are to be accommodated to in effect soften the mount, by eliminating unnecessary damping, and to isolate noise. In contrast, in the second mode, the decoupler is to seat in response to high or large amplitude, low frequency vibration, such as can occur at the resonant frequency of the component. As a result, generally high volume flow is to be generated through the damping orifice track. This provides the necessary high damping rate for engine (or other component) control. In a third mode, generally medium amplitude, low frequency vibration, encountered mainly during normal driving, is to produce intermediate fluid flow through the orifice track and moderate damping.
In a second operative position of the control element, the mount assembly is to provide a fourth mode that furnishes a reduced dynamic rate particularly adapted to provide more complete isolation of low or small amplitude, low frequency vibrations/noise, such as occur during engine idle in a stationary vehicle.
In a third operative position the fifth mode is to furnish significantly improved tuning of low amplitude vibrations in a range of higher frequencies from approximately 10 to 200 Hz.
It would also be desirable for this fifth operational mode to allow the assembly to fully compensate for the changing flow characteristics of the hydraulic fluid that is believed to take place at these relatively higher frequencies; i.e. the fluid transitions from laminar to turbulent flow causing a change in expected operational characteristics. As a result of the turbulent flow, both the decoupler passage and orifice track(s) become restricted, eventually becoming effectively choked off. This prevents continued fluid flow between the chambers that is critical for proper damping and vibration/noise control. The flow cut-off results in a significant pressure buildup in the primary chamber of the mount that causes a very sharp increase in the dynamic rate characteristics. The resulting increase in stiffness caused by the high dynamic rate prevents the best suppression and isolation of low amplitude/relatively high frequency vibrations. A need is therefore identified for a mount assembly providing improved tuning of the higher frequencies; that is, in the range of 10-200, and particularly in the low-to-medium part of the range.